Cat pooping outside litter box

I have had an ongoing issue with my cat where she will poop outside the litter box. Her litter box is in the basement and when she has to go, she makes to the basement, walks around, and then poops on the floor half the time (the other half, in the litter box). Her poops have been soft a well over a year now, and come out as "pattys". I had her tested at the vet and there wasn't anything that they could find. Sometimes, she farts and some feces comes out (which gets on my carpet or bed which is quite frustrating). She does make really weird, loud noises before she goes, however other than that, her behaviour is completely fine. 

I've tried probiotics, changing her diet to wet food (that is additive, and grain free). I have experimented with a bunch of brands of grain free, dry food and still no luck. I clean up her messes with cleaner that is suppose to remove animal enzymes (to hopefully help prevent her from thinking she can use that spot again). 

I am kind of getting at wits end about this as her issue is starting to have an impact on my home (stains on the basement concrete floor, and basement stairs). Not to mention cleaning her poop up almost daily is beyond frustrating. 

Please help!

First of all, I'd ask the vet to run the PCR diarrhea panel just to rule out any parasites.  Then, ask for a sample of a Royal Canin sensitive tummy food.  We feed one of our cats Royal Canin Gastrointestinal HE dry in addition to mostly grain-free wet food.  His system cannot tolerate a totally grain-free diet, so adding just a little bit of the dry crunchies to his diet keeps his poops firm and tootsie-roll like.  Lastly, add one or two more litter boxes and make sure they are the extra big ones.  You can even use a larger Rubbermaid or Sterlite tote.  Some kitties like to pee in one box and poop in a different one. Also, be sure you're scooping twice a day.